What is LIGO?

Aerial photo of LIGO Livingston, Louisiana, showing all of one 4 km long arm and part of the other (off to the right). The visible arms are concrete structures that protect the vacuum tubes from the elements. (Credit: Caltech/MIT/LIGO Lab)

LIGO is the world's largest gravitational wave observatory and a cutting edge physics experiment. Comprising two enormous laser interferometers located thousands of kilometers apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves.

Though it's called an observatory, LIGO is unlike any other observatory on Earth. Ask someone to draw a picture of an observatory and odds are it will look something like the photo below: a typical telescope dome on a mountain-top. As a gravitational wave observatory, LIGO bears no resemblance to this whatsoever, as the photo of the LIGO Livingston intererometer at right clearly illustrates.

The 200-inch Hale Telescope at Palomar Observatory in northern San Diego County, California. (Credit: Tylerfinvold/Wikimedia Commons)

Although LIGO will search for gravitational waves from space, and it is called an "Observatory", LIGO is not, strictly speaking, intended to be solely an astronomical facility. LIGO is truly a physics experiment on the scale and complexity of some of the world's giant particle accelerators and nuclear physics laboratories. Though its mission is to detect gravitational waves from some of the most violent and energetic processes in the Universe, the data it will collect will have far-reaching effects on many areas of physics including gravitation, relativity, astrophysics, cosmology, particle physics, and nuclear physics.

Since LIGO has the word "Observatory" in it, however, it is helpful to first describe how it differs from the observatories that most people envision. Three things truly distinguish LIGO from an astronomical observatory:

First, LIGO is blind. Unlike optical or radio telescopes, LIGO cannot see electromagnetic radiation (e.g., visible light, radio waves, microwaves) nor does it have to because gravitational waves are notpart of the electromagnetic spectrum. In fact, electromagnetic radiation from space is so unimportant to LIGO that it is completely isolated and sheltered from the outside world. LIGO cannot (nor does it need to) see anything. Rather, it 'feels' for invisible gravitational waves.

Second, LIGO is the opposite of round. Since LIGO doesn’t need to collect light from stars or other objects in the Universe, it doesn't need to be dish-shaped like telescope mirrors or radio dishes, which collect and focus electromagnetic radiation to produce images. Rather than having 'eyes' typical of astronomical observatories, LIGO really has ears consisting of two straight and level 4 km (2.5 mi.) long steel vacuum tubes, 1.2 m in diameter, arranged in the shape of an “L”, and protected by a 10-foot wide, 12-foot tall concrete enclosure that protects the tubes from the outside world. (See photos at right.)

Third, LIGO cannot function alone. While an astronomical observatory can function and collect data just fine on its own (some do not, by choice), gravitational wave observatories like LIGO cannot operate solo. The only way to definitively detect a gravitational wave is by operating in unison with a distant twin so that local vibrations are not mistaken for signals from gravitational waves. There are some very good reasons for this, which you can learn about in LIGO’s Dual Detectors.

These are the three most prominent physical differences between LIGO, a gravitational wave observatory, and astronomical observatories.